1. 1 Exotic States 2005 E.C. Aschenauer The search for Pentaquarks at on behalf of the HERMES Collaboration E.C. Aschenauer DESY

2. 2 Exotic States 2005 E.C. Aschenauer HERA: e + /e - (27GeV) - proton collider HERA @ DESY Polarized Deep Inelastic Scattering HERMES @ HERA self-polarised electrons: e ~ 53 ± 2.5 % 27.5 GeV (e  /e  ) ~ 53 ± 2.5 % 1 H → ~ 85 ±3.8 % 2 H → ~ 84 ±3.5 % 1 H ~ 74 ±4.2 % pure nuclear-polarized atomic gas, flipped at 90s time intervals

3. 3 Exotic States 2005 E.C. Aschenauer The HERMES Spectrometer Kinematic Range: 0.02 1 GeV 2 and W > 2 GeV Reconstruction:  p/p < 2%,  < 1 mrad Particle ID:  TRD, Preshower, Calorimeter a 1997: Cherenkov, 1998 a : RICH + Muon-ID Internal Gas Target: unpol: H 2,D 2,He,N,Ne,Kr, Xe He, H, D, H

4. 4 Exotic States 2005 E.C. Aschenauer hadron/positron separation combining signals from: TRD, calorimeter, preshower, RICH Aerogel; n=1.03 C 4 F 10 ; n=1.0014 hadron separation Dual radiator RICH for , K, p Particle Identification  K

5. 5 Exotic States 2005 E.C. Aschenauer direct reconstruction: detection of each decay particle, invariant mass reconstruction Particle Reconstruction Hadron identification: RICH  : 1 - 15 GeV p: 4 – 9 GeV Suppress contamination from  (1116) a p   Define appropriate event topology Invariant Mass calibration ± 2MeV

6. 6 Exotic States 2005 E.C. Aschenauer Results after all cuts add p to invariant mass Peak at: 1528 ± 2.6 MeV  = 8 ± 2 MeV Significance: 3.7 Un-binned fit (root): Results are more stable

7. 7 Exotic States 2005 E.C. Aschenauer The signal and its background Peak at: 1527 ± 2.3 MeV  = 9.2 ± 2 MeV Significance: 4.3 Mixed event background excited  * hyperons (not included in Pythia6) PYTHIA

8. 8 Exotic States 2005 E.C. Aschenauer Is there a  ++ Clear Signal for  (1520) a pK - with acceptance: 1.5% No peak structure for   a pK + Zero counts at 91% CL  + not isotensor  probably isosinglet

9. 9 Exotic States 2005 E.C. Aschenauer generated: M=1540 MeV,  =2 MeV The signal width  + Monte Carlo with complete detector simulation reconstructed: M=1539.5 MeV,  =6.2 MeV Detector resolution: FWHM: 10-14.6 MeV measured: FWHM: 19-24MeV intrinsic width:  = 17 +/- 9 +/- 3 MeV

10. 10 Exotic States 2005 E.C. Aschenauer How real is the  + check for “kinematical reflections ” detector acceptance and cuts (pythia6 MC / Toy MC)   vs   is the   a pentaquark or a previously unobserved   ?   add a fourth hadron is the peak still there ? can we suppress background? can we guess the production process for the   ? What about  - ( )? It would purely be produced in fragmentation. ~ 4 / 1 no clear  - signal strong influence from target remnant to  +

11. 11 Exotic States 2005 E.C. Aschenauer  vs  * + Is peak a new    or a pentaquark state ? If peak is   ⇒ also see a peak in M(  ) B.R.: (   ) / (p  s )  3/2 HERMES preliminary --  *+  *- No peak in   spectrum near 1530 MeV

12. 12 Exotic States 2005 E.C. Aschenauer More on the  + spectra signal / background 2:1 standard cuts applied + K * and  veto signal / background: 1:3 add additional 

13. 13 Exotic States 2005 E.C. Aschenauer Comparison with World Data Kn Mean:

14. 14 Exotic States 2005 E.C. Aschenauer What about the  -- Define appropriate event topology Hadron identification: RICH   : 0.25 - 15 GeV p: 2 – 15 GeV Select events from  (1116) a p  - within ±3  of M  Select events from  (1321) a  - within ±3  of M 

15. 15 Exotic States 2005 E.C. Aschenauer Final spectra for  –- and  0 00 UL for  -- (1860) cross-section: 1.0 - 2.1 nb UL for  0 (1860) cross-section: 1.2 - 2.5 nb Cross-section for  0 (1530): 8.8 – 24 nb Cross section and UL very sensitive to p t and p z distributions Mixed event background

16. 16 Exotic States 2005 E.C. Aschenauer Production cross sections and UL Integrated luminosity: 290 pb -1 all measurements done in quasi-real photo-production (Q 2 <<1GeV 2 ) Acceptance from MC: -  (1520): 1.5% -  + : 0.05% -  0 (1530): 0.036-0.1% -  0 (1860): - 0.065% -  -- (1860): - 0.031% HERMES estimate:  (  (1520)) = 62 ± 11 nb  (  + )= 100-220 nb ± 25% (stat) (additional factor 2 from production kinematics)  (  0 (1530)) = 8.8 – 24 nb Pythia-6 p t and p z spectra or p t and p z spectra from  measurements of HERMES

17. 17 Exotic States 2005 E.C. Aschenauer HERMES contribution to exotic searches Phys. Lett. B 585 (2004) 213 00 hep-ex/0412027

18. 18 Exotic States 2005 E.C. Aschenauer Conclusion and Outlook Direct reconstruction of  + invariant mass Mass: FWHM-Resolution: Significance: ~ 4   + is probably an iso-singlet Additional  improves signal / background no signal observed for  --  and  0 for all cross sections and c.s. U.L determined in quasi-real photo-production (Q 2 <<1 GeV 2 ) Continue data taking for more statistics